In known offshore wind farms, the individual wind turbines are often connected to one another by a simple undersea cable. This arrangement provides that individual stretches of cable within a medium-voltage grid between individual wind turbines of large wind farms cannot be separated. The use of load separators in each wind turbine allows individual wind turbines to be separated from the medium-voltage grid. The wind turbines moreover have power switches in order to control the energy fed into the medium-voltage grid by the wind turbines.
Switchgear of this type (e.g., medium-voltage switchgear) can be provided inside a metal cabinet or housing that seals off the load separators and the power switches in a gastight fashion. In such medium-voltage switchgear, arc faults may form, for example in the case of a malfunction, which heat up the insulating gas and result in high temperatures and high pressures. Causes for the formation of arc faults may be, inter alia, insulation failure, excess voltage or improper operation by staff. The arc faults and resulting hot gases, which may in some cases be toxic, represent a high risk for people standing in the vicinity of the switchgear and for the switchgear itself.
Although metal-enclosed switchgear can withstand several hundred mbar of excess pressure, high internal pressures can cause damage. For these reasons, of such medium switchgear pressure can be relieved from the inside of the housing into the surroundings of the switchgear. To do this, a bursting disk, a pressure-relief valve or the like and possibly an absorbing device, situated in a side or rear housing wall or on the top of the housing, can be provided in the metal cabinet or housing for the switchgear.
When such medium-voltage switchgear is used in a wind turbine, hitherto two options have been known for effecting the pressure relief. Firstly, the pressure is relieved from the medium-voltage switchgear into the interior of a tower of the wind turbine. Any people present inside the tower can be put at risk as a result, and the inside of the tower should be cleaned after an arc fault in order to remove residues of the sometimes toxic decomposition products. If other appliances are located there which are not separated from the atmosphere of the inside of the tower in a gastight fashion, the gases and decomposition products which are formed in the event of an arc fault are also distributed in these appliances, entailing additional expenditure on cleaning. Depending on the degree of contamination of the atmosphere inside the tower, it may also be necessary to specify people to enter the tower only if they are wearing protective clothing, which represents additional expenditure for restoring the wind turbine to operation.
A second option for the pressure relief consists in collecting the gases and decomposition products formed in the event of an arc fault in a duct and then discharging them into the external environment through the external wall of the tower of the wind turbine. However, the opening in the external wall of the tower constitutes an undesired mechanical weakening of the structure. It is also difficult to seal off the external atmosphere in the vicinity of the tower from the interior of the tower or from the inside of the housing for the switchgear. Problems can occur due to the penetration of moisture, such as on the pressure-relief device which forms the separation between the external atmosphere and the interior of the housing of the switchgear. Because the pressure-relief device in offshore wind turbines is permanently exposed to an aggressive marine climate, the components of the pressure-relief device are liable to corrosion.
A switchgear cell arrangement for medium-voltage switchgear is known, for example, from EP 0 620 626 A1 in which, in the event of a disturbance in one of the cells, a relief of pressure can be triggered by pivotably mounted pressure-relief valves in the roof so as to drain off the pressurized gases into the surroundings of the switchgear.
Medium-voltage switchgear is also known from the document U.S. Pat. No. 7,054,143 B2 in which pressurized gases are drained off via a discharge duct attached to the roof of the housing of the switchgear.
Medium-voltage switchgear is known from the document WO98/19377, in which an arc fault absorption device is provided, the absorption device being designed on the rear and/or lateral region of the medium-voltage switchgear in order to discharge pressurized gases or decomposition products.
Furthermore, the document EP 1 477 668 A1 discloses the arrangement of a wind turbine power unit inside a tower of a wind turbine, the wind turbine power unit being arranged on the foundation of the tower.